1. Field of the Invention
The present invention relates to a self-cleaning member and a coating composition.
2. Description of the Related Art
In recent years, how to prevent adhesion of taint is gaining considerable attention in dwelling space and outdoor environment. For example, worsening of living environment and esthetic appearance due to oils and microorganisms such as fungi and algae in the indoor environment, and worsening of esthetic appearance due to the adhesion of taint on the exterior of architecture caused by environmental pollution and the like in the outdoor environment are posing problems. Among these, in the architectural exterior field, taint on the surface of the architectural exterior materials and outdoor construction are posing problems in association with the environmental pollution. Soot and particles suspended in the air deposit on the roof and exterior of the buildings in fine weather. The deposit is carried away by rainwater when it rains and flows down the exterior of buildings. Furthermore, soot suspended in the air is captured by rain and the rain which captures the soot flows down the exterior of buildings and surface of the outdoor construction. As a result, contaminants adhere on the surface along the pass of the rainwater and a streaky taint appear on the surface when such a surface is dried.
Furthermore, the exterior wall of tall buildings becomes tainted by hydrophobic taints such as soot in the air, combustion products such as exhaust gas, taints eluted from the sealant located above and contaminants ejected from the exhaust vent of buildings. These hydrophobic taints are dark-colored and extremely defile the esthetic appearance of buildings. Taints on the architectural exterior material and coated films are considered to be composed of combustion products such as carbon black and inorganic contaminants such as urban soot and clay particles (For example, refer to Journal of Structural and Construction Engineering, Vol. 404. p. 15-24, October 1989, Architectural Institute of Japan).
According to the conventionally accepted idea, a water-repellent paint such as polytetrafluoroethylene (PTFE) was considered to be preferable to prevent the aforementioned taints on architectural exterior and the like. However, since such painted surface tends to get tainted in the shape of the rain flow streaks, it is considered recently that the surface of the coated film should be as hydrophilic as possible against the urban soot which contains hydrophobic components in a large amount (For example, refer to Monthly Functional Materials (Gekkan Kinou Zairyou), Vol. 44, p. 47, August 2005, CMC Publishing Co., Ltd.).
Under these circumstances, there has been recently known a technique to make the surface of the coated film hydrophilic after application by adding a hydrophilic component such as a hydrophilic graft polymer, acrylic silicone, etc. to a coating material. Such coated film exhibits the hydrophilicity of 20 to 60 degree in terms of contact angle of water.
However, since the inorganic dust represented by clay mineral has a contact angle of water of 20 to 50 degree and has an affinity for the aforementioned hydrophilic component that has a contact angle of water of 20 to 60 degree, resulting in the tendency to adhere to the surface, and thus the coated film of the graft polymer cannot prevent taints by the inorganic dust. In addition, it is considered that, on the surface of the coated film with a contact angle of 20 to 60 degree, water drops adhering on the surface by rainfall and the like do not form a uniform water film but are likely to remain on the surface as tiny droplets to which taint components tend to adhere and remain as taints, resulting in spoiling the esthetic appearance. As an antifouling technology to prevent the aforementioned various causes of contamination as well as the contamination by the adhesion of microorganisms such as fungi and algae, there are proposals to make the surface superhydrophilic with a contact angle of water of 20 degree or less. Among these, there is gaining attention a technique to provide an environmentally preferred approach by coating the architectural exterior with a photocatalytic material and by imparting the coated surface with a self-cleaning function by rainfall and a function to decompose harmful gases such as NOx, wherein the coated surface is made hydrophilic by irradiation of sunlight (For example, refer to Japanese Patent Laid-Open Publication No, 2003-342526 and No. 2004-143452).